He told Greene that digital-effects houses needed to understand that each Shoggoth had at least “eight permutations.” He said, “Let’s say that creature A turns into creature A-B, then turns into creature B, then turns into creature B-C. And by the time it lands on a guy it’s creature E.” He discussed one grisly Shoggoth transformation: “It’s like when you grab a sock and you pull it inside out. From his mouth, he extrudes himself.”

And some more:

“Really nice,” del Toro said. “It’s sort of like a tapeworm.”
“Yeah,” Davis said. “When it’s forming, instead of just forming eyes, maybe it’s bubbling like mud, or pudding, so you have these sockets forming but no eyes yet. Then it gets one eye and has this cavernous companion. Mummies always freak me out because they have sockets but no eyes.”

About the city:

“I wanted the whole city to be like an abandoned coral reef,” he said. He showed me an image of a cavernous interior space. Everything was tubular and encrusted with skeletal remains—abandoned tools. “A coral reef is a shitload of skeletons fused together, right? All the technology those creatures have, all their technology is organic. You and I use metals, plastics. These creatures don’t have weapons or chisels. They create other creatures as tools.”

About the Old Ones themselves:

The oceanic motif was particularly evident in the design of the Old Ones. Del Toro’s enthusiasm for the lionfish had endured, and the aliens’ wings echoed their flamboyant fins. In motion, he explained, the Old Ones would appear buoyant—“unbound by gravity.” As the camera tracked them caroming around the city, the viewer would feel disoriented, like a panicked scuba diver inside a cave. “We designed the creatures in such a way that they can go forward or backward, or hang, or be vertical, and they still make sense,” he said.

Gore (or lack of it):

Having read the script, I knew that the body count would be high. (“BAMMMMM!!!!! A massive Shoggoth explodes out from the tower!!!!! It grabs and devours Gordon in mid-sentence!”) But del Toro promised that the film was “not gory.” Victims would be “absorbed” by the aliens in ways that were “eerie and scary.” He explained, “When you watch a documentary of a praying mantis eating the head of its mate, because of the complexity of the mouth mechanism, you’re fascinated. It’s a horrible act, but you’re fascinated.” Though he wouldn’t be spattering blood, he said that he needed to fight Universal for an R rating, “to have the freedom to make it really, really uncomfortable and nasty.”